Dynamic control systems for automotive vehicles have recently begun to be offered on various products. Dynamic control systems typically control the yaw of the vehicle by controlling the braking effort at the various wheels of the vehicle. Yaw control systems typically compare the desired direction of the vehicle based upon the steering wheel angle and the direction of travel. By regulating the amount of braking at each corner of the vehicle, the desired direction of travel may be maintained. Typically, the dynamic control systems do not address roll of the vehicle. For high profile vehicles in particular, it would be desirable to control the rollover characteristic of the vehicle to maintain the vehicle position with respect to the road. That is, it is desirable to maintain contact of each of the four tires of the vehicle on the road.
Vehicle roll characteristics have four different regions based on the magnitude of roll and the roll rate changes. In the small magnitude roll region, which may be called vibration roll region, the vehicle body roll angle with respect to the average road surface is mainly induced by moving on the uneven road surface. For example, the rough surface of the road generates high frequency roll vibration; the potholes or big bumps in the road also induce vehicle vibration roll motion.
In the medium magnitude region, which should be called a handling roll region, the body roll motion with respect to the average surface is mainly induced by the handling maneuver of the vehicle. For example, during hard cornering, the vehicle will generate a relatively large roll motion due to the large lateral acceleration of the vehicle. Noticeably one wheel lifting may happen in this region.
The third roll region, which may be called a safety roll region, is where the relative roll motion of the vehicle exceeds the pre-determined limit and the vehicle starts to roll over if there is no proper counteracting effort applied to the vehicle. In this region, the two wheels at one side of the vehicle may be lifting or diverging from contacting the road. This roll motion is mainly due to the large lateral forces applied at the tires from the road and the large magnitude of the lateral acceleration of the vehicle.
The fourth roll region, which should be called the uncontrollable roll region, is where the rollover of the vehicle is inevitable and the passive safety device such as side airbags are activated.
Vibration roll falls within the working region of the semi-active or active suspensions. Since its lack of control authority, semi-active can never be used to achieve roll attenuation beyond vibration roll region. Although active suspension might be used to take care of both vibration roll and handling roll, the power consumption requirement limits its potential for attenuating a large handling roll, and the suspension dynamics may also limit its fast response to a sudden and big handling roll. In order to achieve roll stiffness adjustment, the active suspension needs to control the vertical stiffness and damping at each corner of the vehicle, which most likely will induce the stiffness changes other than roll stiffness, for example, the heave and pitch stiffness of the vehicle. Hence the active suspensions meet the vehicle performance requirement for achieving vibration roll attenuation.
The handling roll control is also called tilt control (or body roll control), which is required to activate on demand. That is, it only activates when the vehicle is in handling maneuver and it must be turned off as the vehicle comes out of the maneuver, for example, when the vehicle leaves a turn and starts to move straight. If the handling roll is not turned off on straight road driving, the uneven road may cause harsh roll motion since road induced vibration roll is not isolated. The handling roll control must not induce suspension stiffness changes in directions other than the roll direction, for example, heave and pitch stiffness especially when the handling is on an uneven road surface. Since active suspension needs to increase the vertical stiffness at the corners (increasing heave and pitch stiffness) so as to increase roll stiffness, it does not meet this requirement.
The safety roll control needs significant control forces to reduce the tire lateral forces and the lateral acceleration of the vehicle such that potential rollover could be prevented. The brake controls used in roll stability control achieve such on-road rollover prevention. However due to response time limitation in the hydraulic braking system, during initial stage of a rollover event there may not be enough braking pressure available.
It would therefore be desirable to provide a roll stability system that fully utilizes the combined but coordinate control of the active anti-roll bar and active brakes to achieve on-road rollover prevention.